In order to generate a voltage different from a power source voltage, e.g., a voltage higher than the power source voltage, usually, a switching power source using inductance elements or a charge pump type booster using capacitance elements is used.
FIG. 4 is a diagram illustrating an example of a voltage supply circuit using a charge pump type booster. As shown in the figure, in this voltage supply circuit, the booster is composed of plural sections of diodes D1, D2, . . . Dn that are connected in series in the same direction between supply terminal T1 of power source voltage Vcc and output terminal T2 of the boosted voltage, capacitors C1, C2, . . . wherein each has one electrode connected between said diodes and a driving signal is input to the other electrode, a charge pump driver that supplies said driving signal to said capacitors C1, C2, . . . , and output capacitor Cout that smoothens the output voltage.
Corresponding to the input clock signal CLK, the charge pump driver generates two driving voltages with phases inverted from one another at the same frequency as that of clock signal CLK, and they are alternately input to capacitors C1, C2, . . .
By means of the charge pump type booster with the aforementioned constitution, capacitors C1, C2, . . . set as charge pumps perform charging/discharge alternately according to the input driving voltage. As a result, a voltage higher than power source voltage Vcc is obtained from output terminal T2. By setting an appropriate number of sections of booster corresponding to the power source voltage Vcc, a desired high voltage can be generated.
However, for the aforementioned conventional charge pump type booster or switching power source, a large current spike takes place in the switching operation. Consequently, when an analog circuit coexists, noise may be mixed into the analog circuit due to crosstalk.
FIG. 5 is a waveform diagram illustrating the waveform of clock signal CLK and power source current Is. As shown in the figure, in each half period of the clock signal, a switching operation is performed in the charge pump driver. Consequently, current spike results. That is, in the charge pump type booster, noise is generated at a frequency twice that of the clock signal supplied to the charge pump driver.
As said noise cannot be reduced in the operation principle, in order to suppress its influence, it is necessary to change the configuration of the circuit elements or to take other measures for reducing the crosstalk, or to shield against transport of noise. As a result, the circuit constitution becomes complicated, and the cost is increased. This is undesired.
A general object of this invention is to solve the aforementioned problems of conventional methods by providing a type of voltage supply circuit characterized by the fact that it can suppress generation of current spikes in operation, so that it can reduce noise, with a simpler circuit configuration and a lower cost.